1. Field of the Invention
The present invention relates to a plane antenna and more specifically to the technology suitable for antenna formed on a dielectric material substrate to generate circularly polarized wave.
2. Background of the Prior Art
In recent years, a vehicle (mobile) such as an automobile has often been provided with an antenna for GPS (Global Positioning System) in the high frequency band or an antenna for receiving radio waves from satellites for satellite digital broadcasts. Moreover, it is also required for a mobile vehicle to install an antenna for transmitting and receiving radio waves for the ETC (Electronic Toll Collecting) system for automatically collecting tolls on expressways and toll roads and for radio wave beacons of the VICS (Vehicle Information Communication System) for providing vehicle traffic information.
For the GPS radio wave, satellite wave for satellite digital broadcast and ETC radio wave, among the radio waves to be transmitted and received with the mobile vehicle explained above, a circularly polarized wave has been used. A patch antenna (plane antenna) has often been used as an antenna for circularly polarized waves in the related art.
FIG. 1 is a schematic plan view illustrating an example of the plane antenna in the related art and also illustrating a structure of a plane antenna provided in Japanese Patent Application JP-A 2005-102183. The plane antenna illustrated in FIG. 1 can receive a right-hand circularly polarized wave and is constituted by forming, on a dielectric material (transparent film) not illustrated, a square loop antenna (power feeding element) and an independent line conductor (non-power feeding element) 140 which is partly bent to include a first part 140A and a second part 140B and is not connected to the loop antenna 120. The reference numeral 270 denotes a tie conductor as a connecting conductor for connecting power feeding terminals 160, 170 and the loop antenna 120 and the code CP denotes the central point of the loop antenna 120, respectively.
Moreover, as illustrated in FIG. 1, the non-power feeding element 140 is arranged at the area near the external side of the loop antenna 120. In more detail, the first part 140A is arranged in parallel to the loop antenna 120 and the second part 140B is arranged in parallel to the line connecting the intermediate point of the power feeding terminals 160, 170 and the vertex opposing to this intermediate point.
Functions of this non-power feeding element 140 will be explained with reference to the description of the paragraph 0069 of Japanese Patent Application JP-A 2005-102183. A loop antenna 120 not provided with the non-power feeding element 140, particularly a loop antenna 120 having a circumference (total length of the antenna conductor) equal to one wavelength, can receive only the electric field element (horizontal element) in the perpendicular direction (namely, cannot perfectly receive the circularly polarized wave changing the direction of electric field in accordance with time) but can also receive the vertical element of the circularly polarized wave in the case where the non-power feeding element 140 is provided adjacent to the loop antenna 120.
That is, it becomes possible that the vertical element of the circularly polarized wave is received with the second part 140B of the non-power feeding element 140 and the received vertical element is coupled with the antenna conductor of the loop antenna 120 with the first part 140A adjacent to the antenna conductor of the loop antenna 120. As a result, the vertical element and the horizontal element of the circularly polarized wave can be received with the loop antenna 120 in the in-phase state. In other words, if the non-power feeding element 140 is formed of only the second part 140B, the received circularly polarized wave is not easily transferred to the loop antenna 120. Therefore, the first part 140A is provided to the non-power feeding element 140 in order to effectively transfer the received circularly polarized wave to the loop antenna 120.
The technologies proposed, for example, in Japanese Patent Applications JP-A 2005-72716 and JP-A 1997-260925 are also utilized as the antenna structures in the related art. The technology of Japanese Patent Application JP-A 2005-72716 proposes a thin plane structure formed of a plurality of double-loop antenna elements and relates to an antenna structure for simultaneously generating the left-hand circularly polarized wave and the right-hand circularly polarized wave from both directions.
Meanwhile, the technology of Japanese Patent Application JP-A 1997-260925 relates to a structure where a dipole antenna, a loop antenna, and a plane antenna, which are smaller than a square row antenna, are arranged at the internal side thereof within the plane of the antenna in order to provide optimum directivity of respective antennas formed with mutual interferences of a plurality of antennas.
However, it has been difficult for the technology proposed in Japanese Patent Application JP-A 2005-102183 to obtain sufficient circularly polarized wave characteristics because electric field distribution to the non-power feeding element 140 is rather weak due to its structural features. A reason to be considered is that when a line antenna such as a dipole antenna or the like is simply formed on a dielectric material substrate, the beam is mainly formed in the direction along the plane part of the dielectric material substrate and thereby radiation intensity in the direction crossing the plane part of the dielectric material substrate (namely, in the thickness direction) is reduced.
The technology of Japanese Patent Application JP-A 2005-72716 is intended to simultaneously generate a left-hand circularly polarized wave and a right-hand circularly polarized wave. The technology of Japanese Patent Application JP-A 1997-260925 is intended to enable a reduction in size of the antenna by closely or integrally providing a plurality of antennas within a narrow place and to prevent noise from the inside of the vehicle. Namely, Japanese Patent Applications JP-A 2005-72716 and JP-A 1997-260925 are not intended to obtain excellent circularly polarized wave characteristics.